This invention relates to the art of strengthening and mounting thin-walled metal honeycomb structures, such as those that would have utility as a substrate or preheater in controlling emissions from a gasoline or diesel internal combustion engine. Such structures may be formed by extrusion from extrudable particulate metal batch materials which have the property of being able to flow or plastically deform during extrusion, while being able to become sufficiently rigid immediately thereafter so as to maintain their structural integrity, in the manner set forth in U.S. Pat. Nos. 3,790,654 and 4,758,272. Alternatively, the honeycomb structures may be fabricated from thin metal sheets such as by wrapping corrugated and uncorrugated metal sheets into a honeycomb structure as shown in U.S. Pat. Nos. 3,112,184 and 3,444,925. Further, if desired, the honeycomb structures could be made of pleated thin porous sheets of filter material whose layers are interleaved with corrugated or crimped spacers as disclosed in U.S. Pat. Nos. 2,884,091, 2,952,333 and 3,242,649.
The invention particularly relates to the problem of restraining the movement of a heater body within its housing in the direction of gas flow, and to the problem of unwanted heater flexibility when utilized in, and subjected to, the harsh and rigorous conditions experienced in vehicular emissions control systems. When metal honeycombs are utilized in a vehicular exhaust preheater application, the honeycomb is slotted along cell channels to form a serpentine electrical path, which is needed to establish the appropriate resistance for a preheater operation. However, the slotting of the metal honeycomb structure results in a number of detrimental side effects, including unwanted heater flexibility between adjacent portions of the slots. In addition to mechanical flexibility, deformation of the slots due to stresses produced during thermal cycling may cause a closure of the slots, and result in short circuits in the electrical path.
Attempts have been made in the past to remedy the problems that may result from the slotting of honeycomb structures by filling such slots with various materials. For example, high temperature cement (such as disclosed in Brundage et al. U.S. application Ser. No. 07/767,889, filed Sep. 30, 1991, assigned to the assignee of the present application), ceramic plates, and metal spacers coated with insulating coating, have all been tried with varying degrees of success. However, the use of cement, plates or spacers requires the complete filling of the slots, and accordingly a penalty is paid in the form of added mass to the honeycomb, and the disasterous potential of dislodging and loss of slot fillers.
In addition, the precision mounting of a metallic honeycomb structure within an associated housing to restrain excessive movement, particularly in the direction of gas flow, has not been completely satisfactory, due to the variability from piece to piece of each honeycomb structure. Accordingly, it has been virtually impossible to match a honeycomb heater with an enclosing housing, except by custom fitting.
It thus has been an object of the present invention to provide a mounting system which not only functions to strengthen slotted metallic honeycomb structures and thereby eliminate undesirable heater flexibility, but also which does not require the precise matching of the honeycomb heater element with its enclosing housing.